There are many applications where it is desirable to be able to switch an incoming signal to any one of several output terminals. One example of such a system is a shipboard high frequency (HF) radio system wherein it may be desired to convey a signal to be transmitted to a first antenna positioned in a first location or to a second antenna positioned to another location. For example, the antennas may be located at fore and aft locations on the ship. In the past, mechanical and electromechanical type switches have been used to switch a signal to a selected one of a plurality of antennas. These mechanical or electromechanical switches are subject to reliability problems and their switching speed is relatively low. Semiconductive devices provide fast and reliable operation and are therefore often employed instead.
PIN diodes are semiconductor devices which can be made to operate at relatively high radio frequencies and which function essentially as switched resistors, having a high or low resistance value depending upon the value of the biasing characteristics. These devices typically require a relatively large reverse bias voltage to present a high impedance value, and draw a substantial forward current when forward biased to present a low impedance value. In a typical HF (2-30 MHz) system, where the switch is required to transfer 1,000 watts of RF power, the optimum reverse bias voltage for the PIN diode may be -400 volts, and the optimum forward current required to maintain the diode in its low impedance state may be 150 milliamps.
When PIN diodes are employed as switching devices for applications such as those under consideration, their control can introduce problems. Care must be taken to avoid interaction between the RF signals being switched and the relatively high reverse bias voltages or large forward bias currents supplied to the diodes, and between the RF signals and the control signal which is employed to select the bias (forward or reverse) to be applied to the PIN diodes. In addition, means must be provided to ensure that a failure of a device in the bias circuitry will not couple the large bias currents and voltages into the control circuitry, with almost certain catastrophic results.
Current applications involving antenna switching, for example, communications systems which employ state of the art anti-jamming techniques such as frequency-hopping, put an even greater demand on the speed of high power RF switching. Hopping rates of 2 KHz permit a dwell time at each frequency of only 500 microseconds, and it is a reasonable constraint in such a system that a maximum of ten percent of that dwell time be allotted for switching transients and settling. The PIN diodes themselves can switch relatively quickly, in much less than 50 microseconds. Therefore, the bias circuits must be designed to provide switching times compatible with the switching application. Conventional bias circuitry, using a single switching transistor with a current-limiting load resistor cannot achieve fast bias switching, due to the time required to charge or discharge the stray capacitances to a high voltage through a relatively large resistance.